1. Field of the Invention
The present invention relates to a WDM system.
2. Description of the Related Arts
Recently, with an increasing amount of traffic, wavelength-division multiplexing (WDM) system devices have been widely adopted, and a P to P WDM system including a amplifier/repeater (AMP) of several spans as a basic configuration is used.
In addition to the conventional configuration, a long distance transmission, an optical add/drop function, and an optical cross-connect system are requested as new functions. To realize these requests, a compensation node (CN) in which a combination of a demultiplexing unit and a multiplexing unit is provided in a plurality of P to P WDM systems.
FIG. 1 shows a conventional P to P WDM system.
In FIG. 1, “VAT” is short for “variable attenuator”, “SAU” is short for “spectrum analyzer”, “TA” is short for “transmission amplifier”, “LA” is short for “in-line amplifier”, “RA” is short for “reception amplifier”, “OSC” is short for “optical supervisory channel”, “MUX” is short for “multiplexer”, “DMUX” is short for “demultiplexer”, “E/O” is short for “electro-optic transducer”, “O/E” is short for “opto-electronic transducer”, “TERM” is short for “terminal station”, and “ILA” is short for “inline amplifier”.
The transmission terminal station TERM 1 is first described below. When the optical signals of the wavelengths 1 through n are output from the E/O, the optical level of the optical signal of each wavelength is adjusted by the variable attenuator VAT, and input to a multiplexer MUX 1. The multiplexer MUX 1 wavelength-multiplexes the optical signal of each wavelength, generates wavelength-division multiplexed light, and inputs the wavelength-division multiplexed light to a transmission amplifier TA 1. The wavelength-division multiplexed light amplified by the transmission amplifier TA 1 is transmitted to the transmission path. A part of the light is branched and input to a spectrum analyzer SAU. Then, the spectrum analyzer performs feedback controls on the variable attenuator VAT based on the result of the detection of the optical level of the optical signal of each wavelength, and the optical signal of each wavelength is transmitted at the same optical level.
A transmission path is provided with in-line amplifiers ILA 1, 2, and 3 at predetermined intervals, and a wavelength-division multiplexed light attenuated by the transmission through the transmission path is amplified, thereby realizing a long-distance transmission.
In a reception terminal station TERM 2, a reception amplifier RA 11 amplifies received wavelength-division multiplexed light, a demultiplexer wavelength-demultiplexes the light into optical signals of the respective wavelengths, and an opto-electronic transducer O/E transforms the signals into electric signals and receives them. The spectrum analyzer SAU analyzes the wavelength-division multiplexed light amplified by the reception amplifier RA 11, and the result is transmitted to the transmission side along an optical supervisory channel OSC.
With the configuration shown at the lower portion of FIG. 1, the transmission and reception sides are inverted as compared with the upper configuration, and the explanation is omitted here.
The E/O of each terminal station has the wavelengths of 1 through N available in the WDM system. The signal light of the E/O is wavelength-multiplexed by the MUX1 through the VAT prepared for each wavelength of the TERM1. A multiplexed optical signal is amplified by the transmission AMP (TA11). The output signal of the TA11 is monitored by the SAU, and the SAU issues a control signal to the VAT to allow the peak level of each wavelength to match a target level. Thus, the VAT controls the peak power of each wavelength.
The repeater AMPs (LA11, LA12, SA13) of the ILA1, ILA2, ILA3 amplify the signal light degraded in the transmission path are amplified.
The reception AMP (RA11) of TERM2 as a reception unit amplifies the signal light degraded in the transmission path. The amplified signal light is wavelength-demultiplexed by the DMUX1, and input to the O/E of each terminal station. The SAU in the TERM2 has the function of monitoring the output light of the reception AMP (RA11).
The OSC (optical supervisory channel) is used as a control signal among the WDM stations in the WDM system shown in FIG. 1. The control signal and a multiplexed signal have different wavelengths, and the control signal is multiplexed with a signal light wavelength-multiplexed by a coupler, and demultiplexed. In the transmission AMP (TA11), the signal is multiplexed with a WDM signal and transmitted downstream. In each repeater AMP (LA11, LA12, LA13), the signal is demultiplexed from the WDM signal, and terminated. The OSC signal is multiplexed with a WDM signal with necessary information added, and transmitted downstream. In the reception AMP (RA11), the signal is demultiplexed from the WDM signal, and terminated.
The system control data communicated with each WDM station in the OSC can be the setting information, status information and fault information about each wavelength of the TERM1, the status information and fault information about each AMP, and the status information about the OSC. According to the information, the rise of the system, the increase and decrease of a wavelength, the system during the fault of the system is controlled.
When a long-distance transmission is realized, and the number of spans is increased by increasing in-line amplifiers with the configuration shown in FIG. 1, the transmission degradation occurs by the accumulation of ASE light (natural noise light) generated by the AMP connected to a plurality of stages, the accumulation of tilt (wavelength dependence relating to the loss characteristic) among the wavelengths, etc. As a method of avoiding the transmission degradation due to those described above, there is a method of waveform-regenerating each wavelength light DMUXed by the P to P WDM system by a regenerator (REG) unit.
FIG. 2 shows the configuration of the WDM system using the REG unit.
In FIG. 2, only the difference from FIG. 1 is explained below.
Since the degradation by the ASE light accumulation, tilt accumulation, etc. can be all cancelled by adding the REG unit, the characteristic is not degraded and the long-distance transmission can be performed. However, since it is necessary to prepare a REG unit for each wavelength, a high cost is required.
As a method for simultaneously realizing the cancellation of the accumulation of the ASE light and the accumulation of the tilt and the deletion of the REG unit, a compensation node system for directly connecting the MUX and DEMUX sides of the P to P WDM system opposite to each other is adopted.
FIG. 3 shows the configuration of the compensation node system.
In FIG. 3, the compensation node is formed by the CN-T1, CN-R2, CN-T2, and CN-R1.
The long-distance WDM transmission system with the configuration of the compensation node shown in FIG. 3 has the following advantage when it is compared with the configuration of increasing the number of repeater AMPs (in-line AMP) shown in FIG. 1.
One is that the ASE light accumulated in each AMP at the left side of the compensation node is not transmitted to the right of the downstream compensation node by cutting off the ASE light out of the filter band width by the filter characteristic of the DMUX1 in the CN-R1 shown in FIG. 3.
By the control of the VAT unit of the CN-T1 shown in FIG. 3, the output of each wavelength in the output unit of the transmission AMP (TA21) of the CN-T1 is controlled to be kept at a constant level. Thus, the tilt accumulation generated in each AMP in the left WDM system of the compensation node can be canceled.
From the viewpoint of device control, the CN-R1 and CN-T1 shown in FIG. 3 do not communicate the OSC control information, and do not perform cooperative control between the compensation nodes in the left and right sections.
There are several problems about the long-distance WDM transmission using a compensation node by not communicating device management control information and the optical characteristic information between the devices spanning the compensation nodes.
First problem is that when a plurality of P to P WDM systems are connected using a compensation node, the fluctuation of the level of an upstream unit at the system rise and the addition of a wavelength is transmitted to a downstream unit because each channel of the upstream DMUX unit is directly connected to each channel of the downstream MUX unit. At the system rise and the addition of a wavelength from the upstream unit, there can be the case where an unstable status in which a downstream unit starts a rising operation occurs while the optical level of the upstream unit is gradually enhanced.
The second problem is that the OSNR value indicating the optical characteristic for use in discriminating the transmission path degradation in the WDM transmission path includes an error from a true OSNR value each time a compensation node is passed when a spectrum analyzer (SAU) in the device performs a monitoring process.
FIG. 4 is an explanatory view of the deviation of the OSNR value monitored by spanning compensation nodes from the true OSNR value.
Assume that the OSNR measured by the RA13 of the CN-R1 shown in FIG. 4 is S13, and a total value of the ASE light noise of the TA11, LA12, and Ra13 at the left side of the compensation node is N13. Then, assuming that the line width at the prescription of the OSNR is Δλ1, the ASE light out of the band width is removed, but the ASE light immediately below the signal light passes the DEMUX1 as is, and is accumulated as is in the wavelength of the right system of the compensation node. Assume that the OSNR read value when the SAU measures the OSNR in the right RA23 of the compensation node is S23, and the total value of the ASE light generated in the TA21, LA21, and RA21 to the right of the compensation node is N23. The apparent OSNR value read by the RA23 is S23, but actually the ASE light of N13 accumulated in the left system of the compensation node is also included in the signal light. Therefore, it is necessary to determine that the true OSNR value in the R23 is S23 or N13.
The third problem is that, when a plurality of P to P WDM systems are connected through compensation nodes, the ASE is independently corrected in the upstream WDM unit and the downstream WDM unit, but there arises an error in the amount of ASE correction of the downstream WDM unit by the ASE in the filter band passing as is after the passage through the filter of the upstream DMUX unit.
FIG. 5 is an explanatory view of the outline of the leakage ASE in a compensation node.
Assume that the OSNR measured by the RA13 of the CN-R1 shown in FIG. 5 is S13, and a total value of the ASE light noise of the TA11, LA12, and Ral3 at the left side of the compensation node is N13. Then, assume that the line width at the prescription of the OSNR is Δλ1, and the filter band width of the DMUX1 of the CN-R1 is Δλ2. The filter of the DMUX1 cuts off the ASE light out of the band of the Δλ2 in the wavelength demultiplexed by the DMUX1 of the CN-R1, but cannot cut off the ASE light in the band width, and it is transmitted as is to the MUX2 side of the right system of the compensation node.
In the left and right WDM sections of the compensation node, the ASE is corrected in order to keep constant signal light power per wavelength contained in the output signal of each AMP independent of the number of wavelengths. Refer to the patent document 1 for the detailed principle of the ASE correction.
In the long-distance WDM system with the configuration of the conventional compensation node shown in FIG. 3, it is considered that the ASE correction has been completed for each WDM section. However, the ASE light leaking through the CN-R1 is not actually corrected, thereby resulting in insufficient ASE correction.
The present invention aims at solving the above-mentioned problems, and improves the system performance by the long-distance WDM transmission system with the CN configuration by performing:                stabilizing the operation of system rise by mutual communication of device setting information in the CN section (between the CN-R and the CN-T) in the long-distance WDM system in a compensation node;        displaying an OSNR true value in each WDM section by transmitting the OSNR information for each wavelength in the CN section, and discriminating the waveform degradation in the WDM device in the long-distance WDM system in a compensation node; and        optimizing the amount of ASE correction by calculating the amount of leakage ASE light for each wavelength in the CN section, and performing correction in the long-distance WDM system with the CN configuration.Patent Document 1        
Japanese Patent Application Publication No. 2000-232433